It is possible 
- you need a picture of some mountains that you can use -> either take one from the internet, make one by yourself, render it from a 3d program (create a high poly mountain and render a picture of it)
- you need a plane + uv map it
- place them into the level -> best would be when you also add a little bit exponential height fog or particle systems
- you will probably have to do some small changes to the plane -> e.g that it’s a little bit bent,… But it depends on how it will look like in the level
You can see something similar in the “sun temple” demo which you can find on the marketplace (the cliffs outside of the level)